In a conventional tire pressure monitoring device, it receives without wire information on rotational directions of tires outputted from detecting units provided on all the tires, and detects an installation position of each tire based on the rotational direction of the tire, obtained from the rotational direction information, and receiver sensitivity of received wireless signals. Such a device is disclosed in Japanese Patents Laid-open No. 2005-112056, pages 2-6 and all of its drawings.